Nearly every damp problem we are shown in an old house was caused by a repair. Somebody, at some point, put a modern material on a wall that was built to breathe.
How a stone wall stays dry
A solid wall of Bath stone has no cavity and no damp course. It gets wet when it rains and dries out again through its surface, inside and out. Lime mortar and lime plaster let that happen.
What goes wrong
Cement render, gypsum plaster and vinyl paint are all waterproof. Put on a solid wall, they hold the water in. It finds its way out somewhere else: usually at the skirting, as a tide mark and a smell.
If the paint is blistering at knee height, the wall is not rising damp. It is drying in the only place it still can.
How to tell what you have
Tap it. Lime plaster sounds dull and gives a little under a thumbnail; gypsum is hard, pink or grey, and rings. Lime is usually cream all the way through, with hair in it.
What we do
We take the modern plaster off the affected walls, let them dry for a season where we can, and replaster in three coats of lime. It costs more than a skim. It is also the last time you will do it.
